Clean up build process for cross-compiled targets.

Clean up build process for cross-compiled targets.

  * Over time, the process for producing an SBCL build for another
target architecture or OS has become more difficult and less
documented.

  * Move all make.sh argument processing to make-config.sh.

  * Have make-config.sh produce a file, output/build-config,
containing shell commands to set up important build parameters
such as where to find GNUMAKE, what SBCL_XC_HOST is, and so on.

  * Source output/build-config in most of the make*.sh scripts.

  * The practical upshot of all this is that a cross-compile
build is now a matter of alternating between target and host,
calling make-config.sh, make-host-1.sh, make-target-1.sh,
make-host-2.sh, make-target-2.sh, and following up with a
make-target-contrib.sh on the target, and you wind up with a
usable product.  And, as make-config.sh does all of the arg
parsing, you can pass --xc-host= or --dynamic-space-size= or
whatever and it all works out.